1. Field of the Invention
The present invention relates to a display device for displaying a color image.
2. Description of the Related Art
A projector system disclosed in Japanese Laid Open Patent Application 1-502139 (1989) is shown in FIG. 1 schematically as an example of a conventional display device which will be described in the following.
FIG. 1 shows a construction of an example of a conventional display device.
Referring to FIG. 1, after a white light beam emitted from a light source 15 is directed to an R, G and B switching device (referred to as RGB switching device) 14, R, G and B components of the white light beam are output sequentially to a main prism 11 by causing the polarizing angles of the R, G and B components to be switched at high speed by the RGB switching device 14.
The RGB switching device 14 is constituted with a polarizing prism 1 for polarizing an input white light beam to a white light beam having a predetermined polarized light (P-polarized light), a blue filter 2 for reflecting a blue light component of the output white light from the polarizing prism 1 and passing other light components, a red filter 3 for reflecting only a red light component of the light components passed through the blue filter 2, a green filter 4 for reflecting a green light passed through the blue filter 2 and the red filter 3. Each of the liquid crystal polarizer switches 5, 6 and 7, when it is off, rotates polarizing angle of the incident light component reflected from the respective filters 2, 3 and 4 by 90 degrees and allows to pass it, but these polarization rotated light components pass through subsequently and correspondingly provided blue, red and green filters 8, 9 and 10, thus are not used as reading lights. When each of the liquid crystal polarizer switches 5, 6 and 7 is on, the turned on switch allows to pass the incident light as it is (polarization of the incident light is not rotated). Blue, red and green filters 8, 9 and 10 functions similarly to the filters 2, 3 and 4 respectively. Light reflected by the respective filters 8, 9 and 10 are supplied to the main prism 11. By switching the liquid crystal polarizing switches 5, 6 and 7 of the RGB switching device 14 sequentially, the main prism 11 supplies to a light valve 12 only the color light the polarization of which is not rotated by the turned on polarizing switch 5, 6 or 7.
The light valve 12 is referred to as spatial light modulator (SLM) and has a construction shown in FIG. 6.
FIG. 6 is a sectional view showing a construction of exemplary spatial light modulator.
Referring to FIG. 6, the spatial light modulator 20 has a laminated structure of a glass substrate 21, a transparent electrode (ITO) 22, a photo-conductive layer 23, a dielectric mirror 24, a photo-modulator layer 25 (utilizing birefringence effect of, for example, TN liquid crystal or perpendicularly oriented liquid crystal, etc.), a transparent electrode 26 and a glass substrate 27, in the order. The photo-modulator layer 25 is sandwiched between aligning films 28 and 29.
In writing an image, a voltage is applied across the transparent electrodes 22 and 26 to generate an electric field across the photo-conductive layer 23, and a writing light (image light) is directed to the photo-conductive layer through the glass substrate 21 and the transparent electrode 22. Upon receiving the writing light electric resistance or impedance of the photo-conductive layer 23 varies correspondingly with a sectional intensity distribution of the writing light.
In reading the image, a reading light is directed to the photo-modulator layer 25 through the glass substrate 27 and the transparent electrode 26. The reading light which passes twice the photo-modulator layer 25 being reflected by the dielectric mirror 24 is modulated correspondingly with a state of the writing light, thus read out through the glass substrate 27.
In this case, when the R, G and B lights sequentially supplied from the RGB switching device 14 through the main prism 11 and an image output on a screen of a CRT 13 are supplied to the light valve 12 which is the spatial light modulator 20 as the reading light and the writing light respectively, the image on the screen of the CRT 13 is observable as a color image through the main prism 11 as follows. By sequentially outputting (reading) R, G and B images from the screen of the CRT 13 in synchronism with time divisional and sequential supplies of R, G and B reading lights through the liquid crystal polarizing switches 5, 6 and 7 which are switched at high speed, an observer sees the respectively colored images as a normal color image.
Although a white light is a composition of R, G, B lights, the respective light amounts of the R, G and B components are different from each other and the respective wavelength thereof are also different from each other. Therefore, when the R, G and B reading light beams are supplied in time-divisionally and sequentially to the light valve 12, the light amount and the wavelength spectrum of the reading light vary time to time.
Further, although the dielectric mirror 24 of the light valve 12 (spatial light modulator 20) can reflect the reading light whose wavelength is in a range from 400 to 700 nm, it can not reflect 100% of the incident light, therefore, leakage of the light penetrating into the photo-conductive layer 23 is always present although the amount thereof is very small. Due to such leakage light, an impedance of the photo-conductive layer 23 is affected accordingly, which causes an operating point of the photo-modulator layer 25 to be varied compared with a case of no such leakage light.
R, G and B reading lights supplied to the spatial light modulator 20 (12) are different in light amount and wavelength spectrum from each other. Further, the photo-conductive layer 23 has a sensitivity response which is higher to a visible light of longer wavelength as shown in FIG. 4, when R, G and B lights input to the photo-conductive layer 23 are identical in light amount. Therefore, for the R light to which the photo-conductive layer 23 is highly sensitive or for large amount of light of any color, the photo-conductive layer 23 becomes more sensitive than to others, which causes operating point of the photo-conductive layer 23 to vary responsive to respective colored lights.
If the respectively colored images are projected onto a screen (not shown), it is impossible to obtain a normal white image nor properly color balanced image, causing contrast ratio of the image to be degraded or the projected image may have a reddish coloration, thus a desired, composed color image can not be obtained.